Ablation of the ATP-binding Cassette Transporter, Abca2 Modifies Response to Estrogen-based Therapies

The ATP-binding cassette transporter 2 (ABCA2) is an endolysosomal protein expressed in oligodendrocytes and Schwann cells, prostate, ovary and macrophages. In cell cultures, ABCA2 over-expression has been linked with resistance to the anticancer agent, estramustine phosphate (EMP; a nor-nitrogen mustard conjugate of estradiol). The present study shows that Abca2 knockout (KO) mice have greater sensitivity to a variety of side effects induced by EMP treatment. Chronic EMP (12×100 mg/kg body weight) produced mortality in 36% of KO mice, but only 7% of age-matched wild type (WT). Side effects of the drug were also more prevalent in the KO mouse. For example, during the first week of EMP treatments, 67% of KO males (compared to 6% of WT males) responded with episodic erectile events. In WT mice, ABCA2 protein localized within pene corpuscles, (which rely on modified Schwann cells for amplification of tactile signals) suggesting that the transporter may function in the erectile process. Endothelial nitric oxide synthase (eNOS; a source of nitric oxide during erectile response) levels were similar in WT and KO male penile tissue. Treatment with 100 mg/kg EMP (once daily for four days) elevated serum estradiol and estrone in both WT and KO. However, the circulating levels of these estrogens were higher in KO mice implying a reduced plasma clearance of estrogens as a consequence of ABCA2 ablation. Consistent with the pro-convulsant effects of estrogens, KO mice also displayed an increased incidence of seizures following EMP (14% vs. 0%). Taken together, these data indicate that ABCA2 deficiency renders mice more sensitive to EMP treatment-induced effects implying that the transporter has a role in regulating EMP transport and/or metabolism.